Shaft made of fiber-reinforced composite material

ABSTRACT

Provided is a shaft made of a fiber-reinforced composite material including: a straight layer having a reinforcement fiber oriented in an axial direction; a first reinforcing layer provided outside the straight layer and having a first carbon fiber; and a second reinforcing layer provided outside the first reinforcing layer and having a second carbon fiber, wherein a tensile elastic modulus of the first carbon fiber and a tensile elastic modulus of the second carbon fiber are different from each other and are smaller than that of the reinforcing fiber in the straight layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a shaft made of a fiber-reinforced composite material.

2. Related Background Art

There is a shaft made of a fiber-reinforced composite material used in a golf club shaft or the like. Preferably, such a shaft is reduced in weight and maintains bending strength.

In a shaft made of a fiber-reinforced composite material disclosed in JP 9-141754 A, a reinforcing layer is provided on a straight layer to compensate for the lack of bending strength due to the weight reduction.

SUMMARY OF THE INVENTION

In order to obtain a desired bending rigidity, however, it is preferred to adjust the bending rigidity of a shaft. In the shaft made of the fiber-reinforced composite material disclosed in JP 9-141754 A, since the bending rigidity is determined by a reinforcing layer to be used, the adjustment of the bending rigidity is difficult.

An aspect of the invention is to provide a shaft made of a fiber-reinforced composite material having a structure in which the bending rigidity can be adjusted.

A shaft made of a fiber-reinforced composite material according to an aspect of the invention includes: a straight layer having a reinforcement fiber oriented in an axial direction; a first reinforcing layer provided outside the straight layer and having a first carbon fiber; and a second reinforcing layer provided outside the first reinforcing layer and having a second carbon fiber. A tensile elastic modulus of the first carbon fiber and a tensile elastic modulus of the second carbon fiber are different from each other and are smaller than that of the reinforcing fiber in the straight layer.

In the shaft made of the fiber-reinforced composite material, the tensile elastic modulus of the first carbon fiber in the first reinforcing layer and the tensile elastic modulus of the second carbon fiber in the second reinforcing layer are different from each other. Therefore, it is possible to adjust the bending rigidity of the shaft made of the fiber-reinforced composite material by changing a ratio of the lamination number of the first reinforcing layers to the lamination number of the second reinforcing layers.

The tensile elastic modulus of the second carbon fiber may be smaller than that of the first carbon fiber. In this case, the layer containing the carbon fiber having a lower elastic modulus is located at the outside. However, there is known that a bending fracture of the shaft is caused from the compression side. In addition, as the tensile elastic modulus of the carbon fiber contained in the layer is lower, a compression fracture does not easily occur. Therefore, when the tensile elastic modulus of the second carbon fiber is smaller than that of the first carbon fiber, the compression fracture is suppressed and thus bending strength can be further improved.

The tensile elastic modulus of the first carbon fiber and the tensile elastic modulus of the second carbon fiber may be 10 to 200 GPa. Even when the tensile elastic modulus of the first carbon fiber and the tensile elastic modulus of the second carbon fiber are in the range of 10 to 200 GPa, the bending rigidity can be adjusted as well as the bending strength can be improved.

According to the invention, it is possible to adjust the bending rigidity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view schematically illustrating a shaft made of a fiber-reinforced composite material according to an embodiment;

FIG. 2 is a diagram illustrating a planar shape of a mandrel and a cut shape of prepreg used in each layer of the shaft made of the fiber-reinforced composite material illustrated in FIG. 1;

FIG. 3 is a cross-sectional view schematically illustrating a shaft made of a fiber-reinforced composite material according to a modified example;

FIG. 4 is a diagram illustrating a planar shape of a mandrel and a cut shape of prepreg used in each layer of the shaft made of the fiber-reinforced composite material illustrated in FIG. 3;

FIG. 5 is a cross-sectional view schematically illustrating a shaft made of a fiber-reinforced composite material according to another modified example;

FIG. 6 is a diagram illustrating a planar shape of a mandrel and a cut shape of prepreg used in each layer of the shaft made of the fiber-reinforced composite material illustrated in FIG. 5; and

FIG. 7 is a schematic block diagram of a test apparatus of shafts made of a fiber-reinforced composite material according to Examples 1 to 5 and Comparative Examples 1 and 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the invention will be described in detail below with reference to the accompanying drawings. In the description of the drawings, the same reference numerals are used for the same or equivalent components and the duplicate description will not be presented.

FIG. 1 is a cross-sectional view schematically illustrating a shaft made of a fiber-reinforced composite material according to an embodiment. As illustrated in FIG. 1, a shaft 1 made of a fiber-reinforced composite material is a long tubular body extending in an axial direction AX and is used for a golf club shaft, for example. For example, a width of the shaft 1 made of the fiber-reinforced composite material gradually decreases toward a tip end 1 b from a butt end 1 a. When the shaft 1 made of the fiber-reinforced composite material is used for the golf club shaft, the butt end 1 a corresponds to an end portion of a grip side and the tip end 1 b corresponds to an end portion of a head side.

The shaft 1 made of the fiber-reinforced composite material includes a bias layer 3, a straight layer 4, a first reinforcing layer 5, and a second reinforcing layer 6.

The bias layer 3 is the innermost layer of the shaft 1 made of the fiber-reinforced composite material and is provided over a full length in the axial direction AX of the shaft 1 made of the fiber-reinforced composite material. The bias layer 3 includes a first bias layer 31 and a second bias layer 32. In the bias layer 3, the first bias layer 31 and the second bias layer 32 are alternatively laminated for each of one layer or plural layers. A set of bias layers 3 may have a lamination number of, for example, one to ten layers or two to eight layers. Here, the lamination number represents how many specific layers such as the bias layer 3 are laminated on average, that is, how many times the layers are wound around an axis of the shaft 1 made of the fiber-reinforced composite material, and the lamination number may be the decimal number.

The first bias layer 31 is a positive bias layer. The first bias layer 31 is constituted by prepreg having a reinforcement fiber oriented so as to be inclined with respect to the axial direction AX. The first bias layers 31 are laminated such that the orientation direction of the reinforcement fiber inclines at an angle of, for example, 15° to 75°, 20° to 60°, or 30° to 50° with respect to the axial direction AX.

The second bias layer 32 is a negative bias layer. The second bias layer 32 is constituted by prepreg having a reinforcement fiber oriented so as to be inclined with respect to the axial direction AX and intersect with the orientation direction of the reinforcement fiber of the first bias layer 31. The second bias layers 32 are laminated such that the orientation direction of the reinforcement fiber inclines at an angle of, for example, 45° to −75°, −20° to −60°, or −30° to −50° with respect to the axial direction AX. Here, the positive bias layer is the prepreg which is wound counterclockwise from a large-diameter side to a small-diameter side of the shaft 1 made of the fiber-reinforced composite material, that is, laminated in a left-hand screw direction, and the negative bias layer is the prepreg which is wound clockwise from the large-diameter side to the small-diameter side of the shaft 1 made of the fiber-reinforced composite material, that is, laminated in a right-hand screw direction.

As the prepreg constituting the first bias layer 31 and the second bias layer 32, unidirectional prepreg or fabric prepreg is used. The unidirectional prepreg is easy to control an orientation angle compared to the fabric prepreg. The reinforcement fiber of the first bias layer 31 and the second bias layer 32 may have a tensile elastic modulus of, for example, about 200 to 1000 GPa or about 400 to 800 GPa. As such a reinforcement fiber, a carbon fiber is used by reason of a light weight and a high tensile elastic modulus. As the carbon fiber, any of a polyacrylonitrile (PAN)-based carbon fiber and a pitch-based carbon fiber may be used. In place of the carbon fiber, a metal fiber, a silicon carbide fiber, an alumina fiber, a boron fiber, a potassium titanate fiber or the like may be used.

The straight layer 4 is provided outside the bias layer 3. For example, the straight layer 4 is provided over the full length in the axial direction AX of the shaft 1 made of the fiber-reinforced composite material along an outer peripheral surface of the bias layer 3. The straight layer 4 is constituted by prepreg having a reinforcement fiber oriented in the axial direction AX. That is, the straight layers 4 are laminated such that the orientation direction of the reinforcement fiber inclines at an angle of, for example, 0° to ±5° with respect to the axial direction AX. The lamination number of the straight layers 4 may be, for example, from one layer to ten layers or from two layers to eight layers. The straight layer 4 may different in a laminated thickness between the butt end 1 a side and the tip end 1 b side, but may have the same laminated thickness.

As the prepreg constituting the straight layer 4, unidirectional prepreg is used. The reinforcement fiber of the straight layer 4 may have a tensile elastic modulus of, for example, about 200 to 600 GPa or about 230 to 300 GPa. As such a reinforcement fiber, a carbon fiber is used by reason of a light weight. As the carbon fiber, any of a PAN-based carbon fiber and a pitch-based carbon fiber may be used. In place of the carbon fiber, a metal fiber, a silicon carbide fiber, an alumina fiber, a boron fiber, a potassium titanate fiber or the like may be used.

The first reinforcing layer 5 is provided outside the straight layer 4. For example, the first reinforcing layer 5 is provided at the tip end 1 b side of the shaft 1 made of the fiber-reinforced composite material along an outer peripheral surface of the straight layer 4. The first reinforcing layer 5 is constituted by prepreg having a first carbon fiber of a low tensile elastic modulus. For example, the first carbon fiber is oriented in substantially parallel (for example, about 0° to ±5°) with the axial direction AX. The lamination number of the first reinforcing layers 5 is determined according to the purpose and is, for example, from one layer to ten layers. The lamination number of the first reinforcing layers 5 is one to ten layers at the small-diameter side and is zero to one layer at the large-diameter side, and thus the lamination number thereof may be reduced at the large-diameter side.

As the prepreg constituting the first reinforcing layer 5, unidirectional prepreg or fabric prepreg is used. For example, the prepreg constituting the first reinforcing layer 5 may be formed by laminating about one to five sheets having the identical or non-identical cut shape. The carbon fiber of the first reinforcing layer 5 has a smaller tensile elastic modulus than the reinforcement fiber of the straight layer 4 and has the tensile elastic modulus of, for example, about 10 to 200 GPa. The carbon fiber of the first reinforcing layer 5 may have the tensile strength of, for example, 0.5 to 10 GPa, 0.8 to 10 GPa or 1 to 10 GPa to improve bending strength. As an example of the carbon fiber, a pitch-based carbon fiber is used. Please note that the tensile elastic modulus and tensile strength of carbon fiber is measured in accordance with the JIS R 7608 ‘Carbon fibre—Determination of tensile properties of resin-impregnated yarn’.

The second reinforcing layer 6 is provided outside the first reinforcing layer 5. For example, the second reinforcing layer 6 is provided at the tip end 1 b of the shaft 1 made of the fiber-reinforced composite material along an outer peripheral surface of the first reinforcing layer 5. The second reinforcing layer 6 is constituted by prepreg having a second carbon fiber of a low tensile elastic modulus. For example, the second carbon fiber is oriented in substantially parallel (for example, about 0° to ±5°) with the axial direction AX. The lamination number of the second reinforcing layers 6 is determined according to the purpose and is, for example, from one layer to ten layers. The lamination number of the second reinforcing layers 6 is one to ten layers at the small-diameter side and is zero to one layer at the large-diameter side, and thus the lamination number thereof may be reduced at the large-diameter side.

As the prepreg constituting the second reinforcing layer 6, unidirectional prepreg or fabric prepreg is used. For example, the prepreg constituting the second reinforcing layer 6 may be formed by laminating about one to five sheets having the identical or non-identical cut shape. The carbon fiber of the second reinforcing layer 6 has a smaller tensile elastic modulus than the reinforcement fiber of the straight layer 4 and may have the tensile elastic modulus of, for example, 10 to 200 GPa or about 30 to 200 GPa. The carbon fiber of the second reinforcing layer 6 may have the tensile strength of, for example, 0.5 to 10 GPa, 0.8 to 5 GPa or 1 to 3 GPa to improve the bending strength. The tensile elastic modulus of the second reinforcing layer 6 differs from the tensile elastic modulus of the first reinforcing layer 5. The tensile elastic modulus of the second reinforcing layer 6 may be smaller than that of the first reinforcing layer 5 to further improve the bending strength. As an example of the carbon fiber, a pitch-based carbon fiber is used.

The first reinforcing layer 5 and the second reinforcing layer 6 are located at the small-diameter side (tip end 1 b side) of the shaft 1 made of the fiber-reinforced composite material. A lower limit of a range occupied by the first reinforcing layer 5 and the second reinforcing layer 6 may be, for example, not less than 1/20 or not less than 1/10 of the full length of the shaft 1 made of the fiber-reinforced composite material from the tip end 1 b. An upper limit of the range occupied by the first reinforcing layer 5 and the second reinforcing layer 6 may be, for example, not more than ⅔ or not more than ½ of the full length of the shaft 1 made of the fiber-reinforced composite material from the tip end 1 b. The length of the first reinforcing layer 5 and the second reinforcing layer 6 may be, for example, about ⅕ to ⅓ of the full length of the shaft 1 made of the fiber-reinforced composite material from the tip end 1 b.

When the range occupied by the first reinforcing layer 5 and the second reinforcing layer 6 exceeds the above range, a reinforcing process is performed even on a portion which does not require reinforcement, and thus a total weight of the shaft 1 made of the fiber-reinforced composite material is increased. In addition, when the range occupied by the first reinforcing layer 5 and the second reinforcing layer 6 does not reach the above range, there is a concern that the tip end 1 b side of the shaft 1 made of the fiber-reinforced composite material is insufficient in bending strength.

Next, a method of manufacturing the shaft 1 made of the fiber-reinforced composite material will be described. FIG. 2 is a diagram illustrating a planar shape of a mandrel and a cut shape of prepreg used in each layer of the shaft 1 made of the fiber-reinforced composite material. As illustrated in FIG. 2, first, the reinforcement fiber used in each layer is formed as prepreg. Each prepreg is formed in the form of sheet where each reinforcement fiber is impregnated with a matrix resin.

As the matrix resin used in each prepreg, a thermosetting resin such as epoxy resin, unsaturated polyester resin, phenolic resin, silicone resin, polyurethane resin, urea resin, melamine resin, or the like is used. A mass per unit area of the reinforcement fiber of the prepreg is not particularly limited, but may be, for example, in the range of 30 to 180 g/m² or 50 to 150 g/m². When the mass per unit area of the reinforcement fiber of the prepreg is larger than the above range, flexibility is limited in the cut shape and weight design of the shaft and winding properties of the prepreg around a mandrel M may be inferior at the time of manufacturing the shaft 1 made of the fiber-reinforced composite material.

Subsequently, each prepreg is cut according to the lamination number and the laminated range (occupied range) of each layer. For example, prepreg P31 of the first bias layer 31, prepreg P32 of the second bias layer 32, and prepreg P4 of the straight layer 4 each have a trapezoidal cut shape having the same height as the full length of the shaft 1 made of the fiber-reinforced composite material. For example, prepreg P5 of the first reinforcing layer 5 and prepreg P6 of the second reinforcing layer 6 each have a triangular cut shape.

Subsequently, the prepreg P31 of the first bias layer 31 and the prepreg P32 of the second bias layer 32 are attached by being shifted from each other in a half length of the circumference of the mandrel M (core bar) made of metal and are wound around the mandrel M as the prepreg of the bias layer 3. Then, the prepreg P4 of the straight layer 4 is wound around the outside of the prepreg of the bias layer 3 which is previously wound. Then, the prepreg P5 of the first reinforcing layer 5 is wound around the outside of the prepreg P4 of the straight layer 4 which is previously wound, and the prepreg P6 of the second reinforcing layer 6 is wound around the outside of the prepreg P5 of the first reinforcing layer 5 which is previously wound.

Thereafter, a heat shrinkage tape is wound around the outside of the prepreg P6 of the second reinforcing layer 6, the prepreg P5 of the first reinforcing layer 5, and the prepreg P4 of the straight layer 4 to fix each layer. In this state, the whole is put into a heating furnace to heat, and thus the matrix resin contained in each layer is cured. Then, after cooling up to room temperature, the heat shrinkage tape is removed and the mandrel M is pulled out, and thus shaft 1 made of the fiber-reinforced composite material is produced.

In the shaft 1 made of the fiber-reinforced composite material configured as described above, the tensile elastic modulus of the carbon fiber in the first reinforcing layer 5 is different from that of the carbon fiber in the second reinforcing layer 6. As the tensile elastic modulus of the carbon fiber becomes larger, the bending rigidity increases. For this reason, when the tensile elastic modulus of the carbon fiber in the first reinforcing layer 5 is different from that of the carbon fiber in the second reinforcing layer 6, it is possible to adjust the bending rigidity of the shaft 1 made of the fiber-reinforced composite material by changing a ratio of the lamination number of the first reinforcing layers 5 to the lamination number of the second reinforcing layers 6.

In addition, the tensile elastic modulus of the carbon fiber in the first reinforcing layer 5 and the tensile elastic modulus of the carbon fiber in the second reinforcing layer 6 are smaller than that of the reinforcement fiber in the straight layer 4. Further, the tensile elastic modulus of the second carbon fiber may be lower than that of the first carbon fiber. Here, when attention is paid to compression fracture strain of the first carbon fiber in the first reinforcing layer and compression fracture strain of the second carbon fiber in the second reinforcing layer, as the tensile elastic modulus becomes smaller, each numerical value of them becomes larger. In other words, as the elastic modulus of the carbon fiber becomes lower, the carbon fiber has characteristics that the compression fracture hardly occurs (here, when comparing the compression fracture strain of the carbon fiber, the value is as follows: Nippon Graphite Fiber XN-05: 2.9%, XN-10: 2.1%, XN-15: 1.75%, Mitsubishi Rayon TR50S: 1.2%, and Nippon Graphite Fiber XN-60: 0.15%). When a bending load is applied to the shaft made of the fiber-reinforced composite material, compression stress concentration occurs at just below an indenter and compression fracture easily occurs from this portion. In other words, bending strength of the shaft made of the fiber-reinforced composite material is influenced by the compression fracture strain. Thus, as the carbon fiber having a lower elastic modulus, that is, the carbon fiber having a higher compression fracture strain is disposed at the outermost portion, it is possible to achieve an effect that the bending strength of the shaft made of the fiber-reinforced composite material is improved. Here, when the carbon fiber having the lower tensile elastic modulus is used in the reinforcing layer to obtain the higher bending strength, the bending strength is improved, but the bending rigidity is also reduced and thus the adjustment range of the bending rigidity also becomes narrower. Meanwhile, it is possible to obtain high bending strength, to obtain moderately high bending rigidity, and to adjust bending rigidity in a wide range by using two types of low elastic carbon fibers, that is, by using the lower elastic carbon fiber having the lower tensile elastic modulus as the second carbon fiber in the second reinforcing layer provided at the outside and using the carbon fiber having the tensile elastic modulus higher than that of the second carbon fiber as the first carbon fiber in the first reinforcing layer provided at the inside.

It is known that the bending fracture of the shaft is caused from the compression side. In addition, as the tensile elastic modulus of the carbon fiber contained in the layer is lower, the compression fracture does not easily occur. Therefore, when the tensile elastic modulus of the carbon fiber in the second reinforcing layer 6 is smaller than that of the carbon fiber in the first reinforcing layer 5, since the layer containing the carbon fiber having the lower elastic modulus is located at the outside, the compression fracture is suppressed and thus the bending strength can be further improved.

The embodiment of the invention is described above, but the invention is not limited to the above embodiment. For example, the shaft 1 made of the fiber-reinforced composite material is used as various shafts as well as the golf club shaft.

In addition, the shaft 1 made of the fiber-reinforced composite material may be further provided with other layers in addition to the bias layer 3, the straight layer 4, the first reinforcing layer 5, and the second reinforcing layer 6. Other layers may be located inside the bias layer 3 or between the layer and the layer. Examples of other layers include a hoop layer having a carbon fiber oriented at an angle of approximately 90° with respect to the axial direction AX, or the like.

Further, the shaft 1 made of the fiber-reinforced composite material may be further provided with at least one reinforcing layer at the outside of the second reinforcing layer 6. The at least one reinforcing layer is constituted by prepreg having a carbon fiber, and the tensile elastic modulus of the carbon fiber is smaller than that of the reinforcement fiber in the straight layer 4.

In the above embodiment, the length of the first reinforcing layer 5 in the axial direction AX is substantially equal to that of the second reinforcing layer 6 in the axial direction AX, but is not limited thereto.

FIG. 3 is a cross-sectional view schematically illustrating a shaft made of a fiber-reinforced composite material according to a modified example. FIG. 4 is a diagram illustrating a planar shape of a mandrel and a cut shape of prepreg used in each layer of the shaft made of the fiber-reinforced composite material illustrated in FIG. 3. As illustrated in FIGS. 3 and 4, a relation between the length of the first reinforcing layer 5 and the length of the second reinforcing layer 6 in the axial direction AX is different as compared to the above embodiment. Specifically, the length of the first reinforcing layer 5 in the axial direction AX from the tip end 1 b is longer than that of the second reinforcing layer 6 in the axial direction AX from the tip end 1 b. Even in this modified example, the same effect as the above embodiment can be achieved.

FIG. 5 is a cross-sectional view schematically illustrating a shaft made of a fiber-reinforced composite material according to another modified example. FIG. 6 is a diagram illustrating a planar shape of a mandrel and a cut shape of prepreg used in each layer of the shaft made of the fiber-reinforced composite material illustrated in FIG. 5. As illustrated in FIGS. 5 and 6, a relation between the length of the first reinforcing layer 5 and the length of the second reinforcing layer 6 in the axial direction AX is different as compared to the above embodiment. Specifically, the length of the first reinforcing layer 5 in the axial direction AX from the tip end 1 b is shorter than that of the second reinforcing layer 6 in the axial direction AX from the tip end 1 b. Even in this modified example, the same effect as the above embodiment can be achieved.

EXAMPLES

The invention will be described in detail below based on Examples and Comparative Examples, but the invention is not limited to the following Examples.

(Prepreg)

Materials used in the shafts made of the fiber-reinforced composite material according to Examples and Comparative Examples are indicated below. Table 1 indicates the materials used in the shafts made of the fiber-reinforced composite material according to Examples and Comparative Examples.

TABLE 1 Prepreg A Prepreg B Prepreg C Prepreg D Prepreg E Name of prepreg E052AA-10N E1026A-09N E1526C-10N NT61250-525S F24N125 Manufacturing company NGF NGF NGF NGF JX Name of CF XN-05 XN-10 XN-15 XN-60 TR50S Manufacturing company NGF NGF NGF NGF MR Tensile elastic modulus of CF Gpa 54 GPa 110 GPa 155 GPa 620 GPa 240 GPa Matrix resin Epoxy resin Epoxy resin Epoxy resin Epoxy resin Epoxy resin Manufacturing company JX JX JX JX JX Curing temperature 130° C. curing 130° C. curing 130° C. curing 130° C. curing 130° C. curing Mass per unit area of CF g/m² 100 g/m² 95 g/m² 100 g/m² 125 g/m² 125 g/m² Resin content, weight % 37 weight % 37 weight % 33 weight % 25 weight % 25 weight % Thickness of prepreg mm 0.109 mm 0.102 mm 0.094 mm 0.093 mm 0.103 mm NGF: Nippon Graphite Fiber Corporation JX: JX Nippon Oil & Energy Corporation MR: Mitsubishi Rayon Co., Ltd.

Prepreg A: carbon fiber prepreg E052AA-10N (made by Nippon Graphite Fiber Corporation, mass per unit area of carbon fiber: 100 g/m², resin content: 37 weight %, thickness of prepreg: 0.109 mm)

Prepreg B: carbon fiber prepreg E1026A-09N (made by Nippon Graphite Fiber Corporation, mass per unit area of carbon fiber: 95 g/m², resin content: 37 weight %, thickness of prepreg: 0.102 mm)

Prepreg C: carbon fiber prepreg E1526C-10N (made by Nippon Graphite Fiber Corporation, mass per unit area of carbon fiber: 100 g/m², resin content: 33 weight %, thickness of prepreg: 0.094 mm)

Prepreg D: carbon fiber prepreg NT61250-525S (made by Nippon Graphite Fiber Corporation, mass per unit area of carbon fiber: 125 g/m², resin content: 25 weight %, thickness of prepreg: 0.093 mm)

Prepreg E: carbon fiber prepreg F24N125 (made by JX Nippon Oil & Energy Corporation, mass per unit area of carbon fiber: 125 g/m², resin content: 25 weight %, thickness of prepreg: 0.103 mm)

The prepreg A is a carbon fiber prepreg obtained by impregnating the carbon fiber of XN-05 (made by Nippon Graphite Fiber Corporation, tensile elastic modulus of 54 GPa, tensile strength of 1.1 GPa) with an epoxy resin (made by JX Nippon Oil & Energy Corporation, curing temperature of 130° C.) as a matrix resin. The prepreg B is a carbon fiber prepreg obtained by impregnating the carbon fiber of XN-10 (made by Nippon Graphite Fiber Corporation, tensile elastic modulus of 110 GPa, tensile strength of 1.7 GPa) with an epoxy resin (made by JX Nippon Oil & Energy Corporation, curing temperature of 130° C.) as a matrix resin. The prepreg C is a carbon fiber prepreg obtained by impregnating the carbon fiber of XN-15 (made by Nippon Graphite Fiber Corporation, tensile elastic modulus of 155 GPa, tensile strength of 2A GPa) with an epoxy resin (made by JX Nippon Oil & Energy Corporation, curing temperature of 130° C.) as a matrix resin.

The prepreg D is a carbon fiber prepreg obtained by impregnating the carbon fiber of XN-60 (made by Nippon Graphite Fiber Corporation, tensile elastic modulus of 620 GPa, tensile strength of 3.4 GPa) with an epoxy resin (made by JX Nippon Oil & Energy Corporation, curing temperature of 130° C.) as a matrix resin. The prepreg E is a carbon fiber prepreg obtained by impregnating the carbon fiber of TR50S (made by Mitsubishi Rayon Co., Ltd., tensile elastic modulus of 240 GPa, tensile strength of 4.9 GPa) with an epoxy resin (made by JX Nippon Oil & Energy Corporation, curing temperature of 130° C.) as a matrix resin.

(Method of Producing Test Pipe)

Hollow CFRP test pipes (Examples 1 to 5 and Comparative Examples 1 and 2) were produced using the above prepreg to simulate the tip end side of the shaft made of the fiber-reinforced composite material.

First, the prepreg was wound around a mandrel made of SUS304 having a diameter of 5.5 mm and a length of 1050 mm in the following order. Prepreg of +45° and pregreg of −45° were attached by being shifted from each other in a half length (8.6 mm) of the circumference of the mandrel and were wound around the mandrel as prepreg of the bias layer. Subsequently, prepreg of the straight layer was wound around the outside of the prepreg of the bias layer which was previously wound. Then, prepreg of the first reinforcing layer was wound around the outside of the prepreg of the straight layer which was previously wound. Furthermore, prepreg of the second reinforcing layer was wound around the outside of the prepreg of the first reinforcing layer which was previously wound.

After the prepreg of each layer was wound, a heat shrinkage tape made of polypropylene and PET (polyethylene terephthalate) was wound around the prepreg of each layer to fix the prepreg. In this state, the whole was put into a heating furnace to heat at 130° C. for one hour, and thus the epoxy resin as a matrix resin was cured. Subsequently, after cooling up to a room temperature, the heat shrinkage tape was removed and the mandrel was pulled out, thereby producing a CFRP pipe having the length of 1050 mm. The CFRP pipe was cut into three equal parts by 350 mm length, thereby obtaining a test pipe having the length of 350 mm.

Example 1

Example 1 had the following lamination structure. The prepreg of +45° was laminated to have two layers and the prepreg of −45° was laminated to have two layers using the prepreg D as the prepreg of the bias layer. The lamination was performed to have three layers using the prepreg E as the prepreg of the straight layer. The lamination was performed to have two layers using the prepreg C as the prepreg of the first reinforcing layer. The lamination was performed to have two layers using the prepreg A as the prepreg of the second reinforcing layer. The thickness of the bias layer was 0.37 ram, the thickness of the straight layer was 0.31 mm, the thickness of the first reinforcing layer was 0.19 mm, and the thickness of the second reinforcing layer was 0.22 mm. The inner diameter of the pipe was 5.5 mm and the outer diameter of the pipe was 7.7 mm.

Example 2

Example 2 had the following lamination structure. The prepreg of +45° was laminated to have two layers and the prepreg of −45° was laminated to have two layers using the prepreg D as the prepreg of the bias layer. The lamination was performed to have three layers using the prepreg E as the prepreg of the straight layer. The lamination was performed to have two layers using the prepreg B as the prepreg of the first reinforcing layer. The lamination was performed to have two layers using the prepreg A as the prepreg of the second reinforcing layer. The thickness of the bias layer was 0.37 mm, the thickness of the straight layer was 0.31 mm, the thickness of the first reinforcing layer was 0.20 mm, and the thickness of the second reinforcing layer was 0.22 mm. The inner diameter of the pipe was 5.5 mm and the outer diameter of the pipe was 7.7 mm.

Example 3

Example 3 had the following lamination structure. The prepreg of +45° was laminated to have two layers and the prepreg of −45° was laminated to have two layers using the prepreg D as the prepreg of the bias layer. The lamination was performed to have three layers using the prepreg E as the prepreg of the straight layer. The lamination was performed to have two layers using the prepreg C as the prepreg of the first reinforcing layer. The lamination was performed to have two layers using the prepreg B as the prepreg of the second reinforcing layer. The thickness of the bias layer was 0.37 mm, the thickness of the straight layer was 0.31 mm, the thickness of the first reinforcing layer was 0.19 mm, and the thickness of the second reinforcing layer was 0.20 mm. The inner diameter of the pipe was 5.5 mm and the outer diameter of the pipe was 7.6 mm.

Example 4

Example 4 had the following lamination structure. The prepreg of +45° was laminated to have two layers and the prepreg of −45° was laminated to have two layers using the prepreg D as the prepreg of the bias layer. The lamination was performed to have three layers using the prepreg E as the prepreg of the straight layer. The lamination was performed to have three layers using the prepreg C as the prepreg of the first reinforcing layer. The lamination was performed to have a single layer using the prepreg A as the prepreg of the second reinforcing layer. The thickness of the bias layer was 0.37 mm, the thickness of the straight layer was 0.31 mm, the thickness of the first reinforcing layer was 0.28 mm, and the thickness of the second reinforcing layer was 0.11 mm. The inner diameter of the pipe was 5.5 mm and the outer diameter of the pipe was 7.6 mm.

Example 5

Example 5 had the following lamination structure. The prepreg of +45° was laminated to have two layers and the prepreg of −45° was laminated to have two layers using the prepreg D as the prepreg of the bias layer. The lamination was performed to have three layers using the prepreg E as the prepreg of the straight layer. The lamination was performed to have a single layer using the prepreg C as the prepreg of the first reinforcing layer. The lamination was performed to have three layers using the prepreg A as the prepreg of the second reinforcing layer. The thickness of the bias layer was 0.37 mm, the thickness of the straight layer was 0.31 mm, the thickness of the first reinforcing layer was 0.09 mm, and the thickness of the second reinforcing layer was 0.33 mm. The inner diameter of the pipe was 5.5 mm and the outer diameter of the pipe was 7.7 mm.

Comparative Example 1

Comparative Example 1 had the following lamination structure. The prepreg of +45° was laminated to have two layers and the prepreg of −45° was laminated to have two layers using the prepreg D as the prepreg of the bias layer. The lamination was performed to have three layers using the prepreg E as the prepreg of the straight layer. The lamination was performed to have four layers using the prepreg B as the prepreg of the first reinforcing layer. The thickness of the bias layer was 0.37 mm, the thickness of the straight layer was 0.31 mm, and the thickness of the first reinforcing layer was 0.41 mm. The inner diameter of the pipe was 5.5 mm and the outer diameter of the pipe was 7.7 mm.

Comparative Example 2

Comparative Example 2 had the following lamination structure. The prepreg of +45° was laminated to have two layers and the prepreg of −45° was laminated to have two layers using the prepreg D as the prepreg of the bias layer. The lamination was performed to have three layers using the prepreg E as the prepreg of the straight layer. The lamination was performed to have four layers using the prepreg E as the prepreg of the first reinforcing layer. The thickness of the bias layer was 0.37 mm, the thickness of the straight layer was 0.31 mm, and the thickness of the first reinforcing layer was 0.41 mm. The inner diameter of the pipe was 5.5 mm and the outer diameter of the pipe was 7.7 mm.

(Static Three-Point Bending Test)

FIG. 7 is a schematic block diagram of a test apparatus of the shafts made of the fiber-reinforced composite material according to Examples 1 to 5 and Comparative Examples 1 and 2. As illustrated in FIG. 7, a test apparatus 50 is an apparatus configured to perform a static three-point bending test and includes a pair of supporting members 51 and an indenter 52. The pair of supporting members 51 is a member configured to support a test pipe 10 of the shaft made of the fiber-reinforced composite material. An upper end of the supporting member 51 forms a curved surface having a radius of 5 mm. A distance between the pair of supporting members 51, that is, a distance between support points is 300 mm. The indenter 52 is a member configured to apply a load to the test pipe 10 of the shaft made of the fiber-reinforced composite material from the top. A lower end of the indenter 52 forms a curved surface having a radius of 75 mm. A distance between each supporting member 51 and the indenter 52, that is, a distance between the support point and a load point is 150 mm. The indenter 52 moves downward to apply the load to the test pipe 10.

The static three-point bending test was performed using the test apparatus 50. Specifically, the test pipe 10 was placed on the pair of supporting members 51, and the load was applied to the test pipe 10 by the indenter 52. The moving speed of the indenter 52 was set to be 1 mm/min. In Examples 1 to 5 and Comparative Examples 1 and 2, a bending fracture load, fracture deflection, bending rigidity, and absorption energy were measured. Table 2 indicates measurement results of the shafts made of the fiber-reinforced composite material according to Examples 1 to 5 and Comparative Examples 1 and 2.

TABLE 2 Comparative Comparative Example 1 Example 2 Example 3 Example 4 Example 5 Example 1 Example 2 Lami- Bias layer Inner Prepreg D (CF: XN-60) nation layer +45°: two layers struc- −45°: two layers ture Four layers in total, full thickness of bias layer 0.37 mm Straight layer Prepreg E (CF: TR50S) 0°: Three layers, full thickness of straight layer 0.31 mm First Prepreg C Prepreg B Prepreg C Prepreg C Prepreg C Prepreg B Prepreg E reinforcing (CF: XN-15) (CF: XN-10) (CF: XN-15) (CF: XN-15) (CF: XN-15) (CF: XN-10) (CF: TR50S) layer 0°: two layers 0°: two layers 0°: two layers 0°: three layers 0°: single layer 0°: four layers 0°: four layers Layer thickness Layer thickness Layer thickness Layer thickness Layer thickness Layer thickness Layer thickness 0.19 mm 0.20 mm 0.19 mm 0.28 mm 0.09 mm 0.41 mm 0.41 mm Second Outer Prepreg A Prepreg A Prepreg B Prepreg A Prepreg A — — reinforcing layer (CF: XN-05) (CF: XN-05) (CF: XN-10) (CF: XN-05) (CF: XN-05) layer 0°: two layers 0°: two layers 0°: two layers 0°: single layer 0°: three layers Layer thickness Layer thickness Layer thickness Layer thickness Layer thickness 0.22 mm 0.22 mm 0.20 mm 0.11 mm 0.33 mm Dimen- Inner diameter mm 5.5 5.5 5.5 5.5 5.5 5.5 5.5 sion of pipe Outer diameter mm 7.7 7.7 7.6 7.6 7.7 7.7 7.7 of pipe Result Bending kgf 45.1 47.5 46.2 48.8 43.1 41.2 40.3 of fracture load bending Fracture mm 32 31.1 26 31.3 32.2 25.8 16.1 test deflection Bending kgf · 9.02 × 10⁵ 8.59 × 10⁵ 1.02 × 10⁶ 9.76 × 10⁵ 8.36 × 10⁵ 9.80 × 10⁵ 1.47 × 10⁵ rigidity mm² Absorption J 7.57 7.6 6.18 7.64 6.93 5.43 3.22 energy

In all of Examples 1 to 5 and Comparative Examples 1 and 2, the reinforcing layer was laminated to have four layers. However, the reinforcing layer (first reinforcing layer and second reinforcing layer) contained two kinds of carbon fibers having a different tensile elastic modulus in Examples 1 to 5, whereas the reinforcing layer contained only one kind of carbon fiber in Comparative Examples 1 and 2. As indicated in Table 2, the bending fracture loads in Examples 1 to 5 were greater than either of the bending fracture loads in Comparative Examples 1 and 2, In addition, the fracture deflections in Examples 1 to 5 were greater than either of the fracture deflections in Comparative Examples 1 and 2. Furthermore, all kinds of absorption energy in Examples 1 to 5 were greater than any kinds of absorption energy in Comparative Examples 1 and 2. As described above, it was confirmed that the bending strength (bending fracture load, fracture deflection, and absorption energy) in Examples 1 to 5 was improved, compared to that in Comparative Examples 1 and 2.

Further more, in Examples 1, 4, and 5, the first reinforcing layer and the second reinforcing layer were formed using the same prepreg, but the ratio of the lamination number was different from each other. Therefore, the bending rigidity in Example 1, the bending rigidity in Example 4, and the bending rigidity in Example 5 were different from each other. Thus, it was confirmed that the bending rigidity can be controlled in a wide range by changing the ratio of the lamination number of the first reinforcing layers to the lamination number of the second reinforcing layers. 

What is claimed is:
 1. A shaft made of a fiber-reinforced composite material, comprising: a straight layer having a reinforcement fiber oriented in an axial direction; a first reinforcing layer provided outside the straight layer and having a first carbon fiber; and a second reinforcing layer provided outside the first reinforcing layer and having a second carbon fiber, wherein a tensile elastic modulus of the first carbon fiber and a tensile elastic modulus of the second carbon fiber are different from each other and are smaller than that of the reinforcing fiber in the straight layer.
 2. The shaft made of the fiber-reinforced composite material according to claim 1, wherein the tensile elastic modulus of the second carbon fiber is smaller than that of the first carbon fiber.
 3. The shaft made of the fiber-reinforced composite material according to claim 1, wherein the tensile elastic modulus of the first carbon fiber and the tensile elastic modulus of the second carbon fiber are 10 to 200 GPa.
 4. The shaft made of the fiber-reinforced composite material according to claim 2, wherein the tensile elastic modulus of the first carbon fiber and the tensile elastic modulus of the second carbon fiber are 10 to 200 GPa. 